Orifice boundary layer suction method and system

ABSTRACT

An air-conveying device for use in an HVAC system having an interior surface and an exterior surface. An air-moving device is arranged and disposed to move air through the air-conveying device adjacent to the interior surface of the air-conveying device. The air-conveying device conveys air having passed through an HVAC heat exchanger. The air-conveying device includes one or more openings disposed and arranged to provide a pressure differential sufficient to cause passage of air through the openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag.

FIELD OF THE INVENTION

The present invention is related to devices for conveying air. In particular, the present invention is directed to a diffuser and/or orifice for use in HVAC heat exchanger units.

BACKGROUND OF THE INVENTION

HVAC systems will typically include a heat exchanger unit having a fan arranged to pass air over a heat exchanger. As air is discharged from the unit, the air typically passes through a diffuser/orifice. The diffuser/orifice is a device that permits the passage of air out of a heat exchanger. Diffuser/orifices are typically fabricated with a geometry, which results in less backpressure and an increased airflow. One drawback of the diffuser/orifice is that the flow at or near the diffuser/orifice surface experiences undesirable flow characteristics. In particular, the fluid near the surface of the diffuser/orifice experiences boundary layer formation. Boundary layer separation may also occur further increasing the thickness of the boundary layer region.

Air passing over flat surfaces may flow in a laminar flow profile. Laminar flow profiles experience low drag and results in larger, desirable flow rates. A boundary layer forms as a result of friction between the air and the surface. The boundary layer thickness is defined as the locus of points where the velocity parallel to the flow surface reaches 99% of the mean stream velocity. Therefore, the thicker a boundary layer is, the less area there is available for flow at maximum velocity when passing through the orifice. Separation of the boundary layer from the surface, called boundary layer separation, causes recirculation and/or turbulent flow. Boundary layer separation is a particular problem in applications where the flow of a fluid diverges. In diffusers, boundary layer separation may occur as air passes out of the unit and experiences an air pressure differential. Diverging flow typically occurs when the flow of air out of the heat exchanger unit is diffused through a diffuser/orifice having a flared outlet. The diverging flow provides a reduction in air pressure, which also reduces backpressure against the fan, but may increase the amount of undesirable turbulent flow and susceptibility to boundary layer separation. Air traveling out of a heat exchanger unit through a diffuser/orifice may experience an adverse pressure gradient along the surface of the orifice. The result is that the boundary layer breaks away or separates from the orifice surface forming a broad pulsating wake.

Another type of known diffuser/orifice device is a cylindrical orifice, which conveys air from spaces within heat exchanger units to outside of the heat exchanger units. In diffuser/orifice devices having a substantially cylindrical geometry, the air passing through the cylinder has a relatively large boundary layer. The large boundary layer is due to the friction between the air and the surface of the cylinder. The shape of the entrance to the cylinder plays a large role in determining the eventual thickness of the boundary layer. A smooth and curved orifice entrance will result in less resistance to fluid flow through the orifice. Sharp edges at the orifice entrance result in increased resistance to flow. This resistance is due to the formation of a large boundary layer region that forms just past the orifice entrance. The large boundary layer is susceptible to boundary layer separation and/or turbulent flow, particularly at larger flow rates. In addition, the cylindrical geometry results in a backpressure against the fan that decreases the quantity of air flowing through the diffuser orifice.

As the boundary layer is drawn away from the surface of the diffuser/orifice, the flow loses at least a part of the laminar flow profile and becomes more turbulent. Turbulent flow has increased drag at the surface, has a lower airflow rate and increases backpressure against the fan. The turbulent flow characteristics of the boundary layer are undesirable for heat exchanger unit applications because a significant amount of energy present in the fluid is lost to aerodynamic drag and recirculation of the turbulent flow. The fan blades may extend at least a portion of the way into the orifice. This extension places the tips of the fan blades within this turbulent region rendering this portion of the blade less efficient and may result in increased fan noise. As a result, the fan requires a greater amount of energy to move the air through the diffuser/orifice.

What is needed is a system and method for decreasing the amount of boundary layer separation and/or turbulent flow occurring in orifice/diffusers in order to more efficiently move air out of a heat exchanger unit.

SUMMARY OF THE INVENTION

The invention includes an air-conveying device for use in an HVAC system having an interior surface and an exterior surface. An air-moving device is arranged and disposed to move air through the air-conveying device adjacent to the interior surface of the air-conveying device. The air-conveying device conveys air having passed through an HVAC heat exchanger. The air-conveying device includes one or more openings disposed and arranged to provide a pressure differential sufficient to cause passage of air through the openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag.

Another embodiment of the invention includes a method for reducing aerodynamic drag in an air-conveying device. The method includes providing an air-conveying device having an interior and exterior surface. A flow of air is provided with an air-moving device from a heat exchanger through the air-conveying device and along the interior surface. Flow of a portion of air through the air-conveying device is permitted from an area of higher pressure air adjacent to the interior surface to an area of lower pressure air adjacent to the exterior surface through openings in the air-conveying device to reduce aerodynamic drag.

An advantage of the present invention is that the air flowing through the diffuser/orifice experiences reduced boundary layer separation because the boundary layer is drawn closer to the surface of the diffuser/orifice. The reduction in boundary layer permits the air to flow through the diffuser/orifice in a substantially laminar flow profile, reducing the backpressure against the fan and decreasing the power required by the fan to exhaust the air out of the heat exchanger unit.

Another advantage of the present invention is that the fan capacity in a heat exchanger unit may be decreased without decreasing the total amount of airflow through the system.

Another advantage of the present invention is that cylindrical diffuser/orifices may be used with substantially the same fan power requirements as diffuser/orifices having a flared geometry, and without the expensive manufacturing costs associated with flared diverging outlet diffuser/orifices.

Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cutaway view of a known diffuser/orifice system.

FIG. 2 schematically illustrates a cutaway view of an embodiment of the present invention.

FIG. 3 schematically illustrates a cutaway view showing airflow through a diffuser/orifice of the present invention.

FIG. 4 schematically illustrates a profile of airflow over a surface of a diffuser/orifice of the present invention.

FIG. 5 schematically illustrates a perspective view of an embodiment of the present invention.

FIG. 6 schematically illustrates a perspective view of an alternate embodiment of the present invention.

FIG. 7 schematically illustrates a cutaway view of a known diffuser/orifice system.

FIG. 8 schematically illustrates a cutaway view of an alternate embodiment of the present invention.

FIG. 9 schematically illustrates a cutaway view showing airflow through an alternate embodiment of a diffuser/orifice of the present invention.

FIG. 10 schematically illustrates a profile of airflow over a surface of an alternate embodiment of a diffuser/orifice of the present invention.

FIG. 11 schematically illustrates a perspective view of an alternate embodiment of the present invention.

FIG. 12 schematically illustrates a perspective view of an alternate embodiment of the present invention

FIG. 13 schematically illustrates a cutaway view of a diffuser/orifice in an outdoor unit according to the present invention.

FIG. 14 schematically illustrates a cutaway view of a diffuser/orifice in an outdoor unit according to an alternate embodiment of the present invention.

Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a known diffuser/orifice for use in a heat exchanger unit. Inlet air 106 is drawn by a fan 104 through a diffusion apparatus 102. The diffusion apparatus 102 has an inlet end 101, having a first diameter and an outlet end 103 having a second diameter. In the arrangement shown in FIG. 1, the first diameter is typically smaller than the second diameter. The inlet air 106 passes into the diffusion apparatus 102 at the inlet end 101 and exhausts from the unit, preferably to the atmosphere, from the outlet end 103. The larger diameter of the outlet end 103 provides a reduction in air pressure for the outlet air 108. In addition, diffusion apparatus 102 includes an interior surface 112 receiving airflow from fan 104 and an exterior surface 110 open to atmosphere pressure. As air flows along the interior surface 112 of the diffusion apparatus 102, a boundary layer 114 is formed. The lower pressure outlet air 108 draws the air in the boundary layer 114 away from the surface, causing boundary layer separation. The result of the boundary layer separation is circulating airflow 120, which has increased drag and reduces the flow rate of outlet air 108. The circulating airflow 120 circulates on itself and may reverse direction at or near the surface of the diffusion apparatus 102. Further, even without boundary layer separation, the presence of a boundary layer results in a resistance to flow and this resistance increases as the boundary layer thickness increases. The presence of the boundary layer has the effect of reducing the area available for fluid flow.

FIG. 2 shows a diffuser/orifice according to an embodiment of the present invention. The diffuser/orifice shown in FIG. 2 includes a diffusion apparatus 202 and fan 104. A diffusion apparatus 202 according to this embodiment of the invention is configured with a geometry that diffuses air passing through the diffusion apparatus 202. As shown in FIG. 2, the diffusion apparatus 202 has a circular cross-section that increases in diameter from the inlet end 101 to the outlet end 103. Although the invention has been described and shown with respect to a circular cross-section, any geometry that is capable of exhausting air out of a heat exchanger unit may be used. The geometry of the diffusion apparatus 202 shown in FIG. 2 includes a geometry that causes a reduction in air pressure as the air is exhausted. This reduction of air pressure reduces the backpressure against fan 104 present in heat exchanger unit applications. The reduction in backpressure reduces the power requirements for fan 104. The inlet air 106 flows into the fan 104 to the inlet end 101 of the diffusion apparatus 202. As the fan 104 moves the air, the air pressure is increased at the inlet end 101 to provide the movement of the air through the diffusion apparatus 202. Outlet air 108 travels from the inlet end 101 through the diffusion apparatus 202. The path of the air includes airflow that follows the contour of interior surface 112. The flow of air along interior surface 112 is in frictional contact with the interior surface 112 and forms a boundary layer 214. The characteristics of the airflow of the boundary layer 214 are preferably at least partially laminar flow. Laminar flow profiles allow the outlet air 108 to exit the diffusion apparatus 202 with a greater reduction in air pressure from inlet end 101 to outlet end 103 and a greater reduction in backpressure against the fan 104 than turbulent flow profiles. The diffusion apparatus 202 according to the invention further includes openings 205 arranged and disposed in the diffusion apparatus 202 to allow passage of air from an interior high-pressure area 310 to an exterior low-pressure area 315. The interior high-pressure area 310 is an area within the diffusion apparatus 202 that has increased air pressure due to movement of the air by the fan 104. The low-pressure area 315 is an area present outside of the diffusion apparatus 202 having a lower pressure than pressure within the diffusion apparatus 202. The low pressure area 315 may be an area either open to outdoor atmospheric pressure or includes an area from which inlet air 106 is drawn. The air traveling over interior surface 112 in boundary layer 214 is drawn into openings 205 due to a pressure differential present between high pressure area 310 and the low pressure area 315. The air drawn from the high pressure area 310 to the low pressure area 315 reduces the thickness of the boundary layer 214, reduces the susceptibility to boundary layer separation and maintains substantially laminar flow as the outlet air 108 leaves the diffusion apparatus 202.

FIG. 3 schematically shows air moving in flow directions 305 through the diffusion apparatus 202. The diffuser/orifice shown in FIG. 3 is configured as shown and described above with respect to FIG. 2. The air near the center of the diffusion apparatus 202 travels in a substantially parallel flow directions 305. The air near the interior surface 112 flows in a flow direction 305 that follows the contour of the interior surface 112. The low-pressure area 315 near the exterior surface 110 of the diffusion apparatus 202 draws the air from the high-pressure area 310 to the interior of the diffusion apparatus 202 to the exterior of the diffusion apparatus 202. The air drawn through openings 205 provide an additional decrease in air pressure as the air travels through the diffusion apparatus 202 and out of the heat exchanger unit. The additional decrease in air pressure decreases the amount of backpressure against the fan, reducing the power requirements for the fan. The amount of power savings by the fan over a diffuser/orifice without openings 205 include power savings up to about 30%, preferably up to about 40%. Backpressure against the fan is due to aerodynamic drag experienced by the air passing through the diffusion apparatus 202. The aerodynamic drag is a force in a direction opposite the flow of air through the diffusion apparatus 202. Aerodynamic drag is a result of friction between the air and the surface of the diffusion apparatus 202 and the loss of fluid momentum due to turbulent flow. The force resulting from the aerodynamic drag increases the amount of power required to convey air through the diffusion apparatus 202, increasing the amount of power required by the fan.

FIG. 4 shows an enlarged view of a portion of the diffusion apparatus 202, as shown and described with respect to FIGS. 2-3, according to the present invention illustrating the reduction in the thickness of the boundary layer 214. FIG. 4 includes interior surface 112 and exterior surface 110. As shown, boundary layer 214 is formed on the surface as outlet air 108 flows over interior surface 112. As the boundary layer air 410 travels over openings 205, a portion of the boundary layer air 410 is drawn from the higher pressure area 310 near the interior surface 112 to the lower pressure area 315 near the exterior surface 110. As the boundary layer air 410 is drawn through the surface, the thickness of the boundary layer 214 is reduced, thereby reducing the thickness of boundary layer 214, decreasing the susceptibility to boundary layer separation and increasing the laminar flow characteristics of the boundary layer 214.

FIG. 5 schematically shows a perspective view of a diffuser/orifice according to the present invention. FIG. 5 shows a diffusion apparatus 202, a fan 104, an inlet air 106 and outlet air 108. Openings 205 are shown with a circular geometry, wherein air is permitted to flow from the high pressure area 310 to low pressure area 315. Openings 205 may be fabricated in the diffusion apparatus 202 using any suitable manufacturing technique, including, but not limited to, cutting, drilling and/or punching. As described above with respect to FIGS. 2-4, the air is moved by fan 104 through diffusion apparatus 202. As the air travels through diffusion apparatus 202, a portion of outlet air 108 is drawn through openings 205. As the air is drawn through openings 205, the thickness of boundary layer 214 (not shown in FIG. 5) is reduced.

FIG. 6 schematically shows a perspective view of a diffuser/orifice according to another embodiment of the present invention. FIG. 6 shows a diffusion apparatus 202, a fan 104, an inlet airflow 106 and outlet airflow 108. Openings 205 are shown with a slot geometry, wherein air is permitted to flow from the high pressure area 310 to low pressure area 315. The slot configuration provides an elongated opening that provides a greater surface area in the direction of flow for which the air may be drawn. As in FIG. 4, openings 205 may be fabricated in the diffusion apparatus 202 using any suitable manufacturing technique, including, but not limited to, cutting, drilling and/or punching. The airflow through diffusion apparatus 202 is substantially the same as shown and described above with respect to FIG. 5.

Although FIGS. 5 and 6 show embodiments of the present invention that include circular and slot geometries, any geometry of opening may be used so long as the opening permits the drawing of air from the interior high pressure area 310 to the exterior low pressure area 315. Additionally, the openings 205 may be positioned along the surface of the diffusion apparatus at any location that provides a reduction in boundary layer thickness.

FIG. 7 shows a known diffuser/orifice having a cylindrical geometry. As shown and described in FIG. 1, the inlet air 106 is moved by fan 104, which increases the pressure of the air entering the diffusion apparatus 702. As the air contacts the interior surface 112 of the diffusion apparatus 702 at inlet end 101, the air forms a circulating airflow 120 having a turbulent flow profile, which reduces the pressure drop as the air travels through the diffusion apparatus 702. In addition, the circulating airflow 120 increases the backpressure against the fan 104. The increased backpressure and reduced pressure drop results in a greater power requirement from fan 104. The cylindrical geometry provides a limited pathway for the air to pass, preventing the pressure from reducing until the air exits the diffusion apparatus at outlet end 103. The exhausting of high-pressure air at outlet end 103 further increases the circulating airflow 120 as the outlet air 108 exits the diffusion apparatus 702.

FIG. 8 shows a diffusion/orifice according to an alternate embodiment of the present invention. The arrangement and operation of FIG. 8 is substantially the same as the arrangement shown and described with respect to FIG. 2. However, unlike FIG. 2, the diffusion apparatus 802 shown in FIG. 8 has a substantially cylindrical geometry, including openings 205. A diffusion apparatus 802 according to this embodiment of the invention is configured with a geometry that passes air through the diffusion apparatus 802 where a portion of the air is drawn through openings 205. In this embodiment, the diameter of inlet end 101 and outlet end 103 is substantially the same. Although the invention has been described and shown with respect to a circular cross-section, any geometry that is capable of exhausting air out of a heat exchanger unit may be used. The geometry of the diffusion apparatus 802 shown in FIG. 8 includes a geometry that conveys air, including, but not limited to, square, rectangular or oval cross-sections. The reduction of air pressure resulting from the air passing through openings 205 reduces the amount of aerodynamic drag through the diffusion apparatus 802. The reduction in aerodynamic drag reduces the power requirements for fan 104.

FIG. 9 schematically shows air-moving in flow directions 305 through the diffusion apparatus 802. The diffuser/orifice is configured as shown and described above, with respect to FIG. 8. The air near the center of the diffusion apparatus 802 travels in a substantially parallel flow directions 305. The air near the interior surface 112 flows in a flow direction 305 that follows the contour of the interior surface 112. The low-pressure area 315 near the exterior surface 110 of the diffusion apparatus 802 draws the air from the high-pressure area 310 at the interior of the diffusion apparatus 802 to the exterior of the diffusion apparatus 802. The air drawn through openings 205 provide an additional decrease in air pressure as the air travels through the diffusion apparatus 802 and out of the heat exchanger unit. The additional decrease in air pressure decreases the amount of aerodynamic drag through the diffusion apparatus 802, reducing the power requirements for the fan 104.

FIG. 10 shows an enlarged view of a portion of the surface of the diffusion apparatus 802, as shown and described with respect to FIGS. 8-9, according to the present invention, illustrating the reduction in the thickness of the boundary layer 214. FIG. 10 includes interior surface 112 and exterior surface 110. As shown, boundary layer 214 is formed on the surface as outlet air 108 flows over interior surface 112. As the boundary layer air 410 travels over openings 205, a portion of the boundary layer air 410 is drawn from the higher pressure area 310 near the interior surface 112 to the lower pressure area 315 near the exterior surface 110. As the boundary layer air 410 is drawn through the surface, the thickness of the boundary layer 214 is reduced, decreasing the susceptibility to boundary layer separation, increasing the effective flow area, and increasing the laminar flow characteristics of the boundary layer 214.

FIG. 11 schematically shows a perspective view of a diffuser/orifice according to an alternate embodiment of the present invention. FIG. 11 shows a diffusion apparatus 802, a fan 104, an inlet air 106 and outlet air 108 arranged as shown and described above with respect to FIG. 5. However, unlike FIG. 5, the geometry of diffusion apparatus 802 is substantially cylindrical. As in FIG. 5, the openings 205 are shown with a circular geometry, wherein air is permitted to flow from the high pressure area 310 to low pressure area 315.

FIG. 12 schematically shows a perspective view of a diffuser/orifice according to another embodiment of the present invention. FIG. 12 shows diffusion apparatus 802 arranged substantially the same as FIG. 11. However, openings 205 in FIG. 12 are shown with a slot geometry, wherein air is permitted to flow from the high pressure area 310 to low pressure area 315. The slot configuration provides an elongated opening that provides a greater surface area in the direction of flow for which the air may be drawn.

Although FIGS. 11 and 12 show embodiments of the present invention that include circular and slot geometries, any geometry of opening may be used so long as the opening permits the drawing of air from the interior high pressure area 310 to the exterior low pressure area 315.

FIG. 13 schematically illustrates a heat exchanger unit 1300 according to an embodiment of the present invention. The heat exchanger unit 1300 includes a diffuser/orifice arranged as shown and described with respect to FIG. 2. Heat exchanger unit 1300 also includes a housing 1310 onto which the diffuser/orifice is attached. The diffuser/orifice includes a flared geometry. Heat exchanger coils 1320 are also attached to the housing 1310. The heat exchanger coils 1320 may be any heat exchanger coils known in the art that provide heat exchange between refrigerant and air. Outdoor air 1330 is drawn by fan 104 through the heat exchanger coils 1320. The inlet air 106 is then directed through the diffuser/orifice and exhausted to the atmosphere as outlet air 108.

FIG. 14 schematically illustrates a heat exchanger unit 1300 according to an embodiment of the present invention. The heat exchanger unit 1300 includes a diffuser/orifice arranged as shown and described with respect to FIG. 8. Like in FIG. 13, heat exchanger unit 1300 also includes a housing 1310 onto which the diffuser/orifice is attached. However, in the embodiment shown in FIG. 14, the geometry of the diffuser/orifice is substantially cylindrical. In addition, heat exchanger coils 1320 are attached to the housing 1310. Outdoor air 1330 is drawn by fan 104 through the heat exchanger coils 1320. The inlet air 106 is then directed through the diffuser/orifice and exhausted to the atmosphere as outlet air 108.

While the invention has been described with respect to a diffuser/orifice, any surface that experiences boundary layer separation in an HVAC system may use the system and method of the present invention, such as centrifugal blower housings. In particular, on the exiting side of the centrifugal blower housing, the decrease in boundary layer thickness may provide an increase in the effective flow area.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. An air-conveying device for use in an HVAC system comprising: a passageway having an interior surface and an exterior surface opposite the interior surface; the passageway being arranged and disposed to permit flow of air through the passageway adjacent to the interior surface upon activation of an air-moving device; a plurality of openings in the passageway extending from the interior surface to the exterior surface, the plurality of openings being disposed and arranged to permit flow of air through the plurality of openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag in the passageway; and wherein the flow of air through the plurality of openings occurs in response to a pressure difference between the interior surface and the exterior surface.
 2. The air-conveying device of claim 1, wherein the flow of air through the openings reduces an amount of power required by an air-moving device to move air through the passageway.
 3. The air-conveying device of claim 1, wherein the plurality of openings each have a substantially rectangular geometry.
 4. The air-conveying device of claim 1, wherein the plurality of openings each have a substantially circular geometry.
 5. The air-conveying device of claim 1, wherein the passageway is a diffuser for a heat exchanger unit in an HVAC system.
 6. The air-conveying device of claim 5, wherein the passageway has a substantially frustoconical geometry.
 7. The air-conveying device of claim 1, wherein the passageway is an orifice for a heat exchanger unit in an HVAC system.
 8. The air-conveying device of claim 7, wherein the orifice has a substantially cylindrical geometry.
 9. A method for reducing aerodynamic drag in an air-conveying device comprising: providing an air-conveying device having an interior surface, and an exterior surface and openings extending between the interior surface and the exterior surface; flowing air with an air-moving device through the air-conveying device; and diverting a portion of airflow through the air-conveying device from an area of higher-pressure air adjacent to the interior surface to an area of lower pressure air adjacent to the exterior surface through the openings in the air-conveying device to increase the laminar flow of the air in the air-conveying device.
 10. The method of claim 9, wherein the diverting of air through the openings reduces the amount of power required by the air-moving device to move air through the air-conveying device.
 11. The method of claim 9, wherein the step of providing an air-conveying device includes locating the openings around the perimeter of the air-conveying device.
 12. The method of claim 9, wherein the step of providing an air-conveying device includes locating the openings adjacent to an exit of the air-conveying device.
 13. The method of claim 9, wherein the air-conveying device is a diffuser for a heat exchanger unit in an HVAC system.
 14. The method of claim 13, wherein the diffuser has a substantially frustoconical geometry.
 15. The method of claim 9, wherein the air-conveying device is an orifice for a heat exchanger unit in an HVAC system.
 16. The method of claim 15, wherein the orifice has a substantially cylindrical geometry.
 17. A heat exchanger unit for an HVAC system comprising: a housing having an interior space; a heat exchanger coil disposed in the housing; an air-moving device disposed in the housing to move air through the heat exchanger coil and the housing; and an air-conveying device being arranged and disposed to permit flow of air from the interior space of the housing upon operation of the air-moving device, the air-conveying device comprising: an interior surface and an exterior surface opposite the interior surface; a plurality of openings extending from the interior surface to the exterior surface, the plurality of openings being disposed and arranged to permit flow of air through the plurality of openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag in the air-conveying device; and wherein the flow of air through the plurality of openings occurs in response to a pressure difference between the interior surface and the exterior surface.
 18. The heat exchanger unit of claim 17, wherein the flow of air through the openings reduces an amount of power required by the air-moving device.
 19. The heat exchanger unit of claim 17, wherein the plurality of openings each have a substantially rectangular geometry.
 20. The heat exchanger unit of claim 17, wherein the plurality of openings each have a substantially circular geometry.
 21. The heat exchanger unit of claim 17, wherein the air-conveying device is a diffuser.
 22. The heat exchanger unit of claim 21, wherein the air-conveying device has a substantially frustoconical geometry.
 23. The heat exchanger unit of claim 17, wherein the air-conveying device is an orifice.
 24. The air-conveying device of claim 23, wherein the orifice has a substantially cylindrical geometry. 